1. Field of the Invention
This invention relates to a method for compressing an image signal, which carries gradation, with interpolation encoding processing.
2. Description of the Prior Art
In recent years, image signals carrying gradation are stored in the form of digital signals on a storage medium, such as an optical disk. When necessary, a digital signal is read from the storage medium and used in the reproduction of a visible image on a cathode ray tube (CRT) or the like. Also, an image signal is transmitted in the form of digital signals between facsimile devices, or the like, and a visible image is reproduced from the received image signal on the signal receiving side.
In cases where an image signal is stored or transmitted in the form of a digital signal, image signal compressing and extending techniques are employed so that the cost required for the digital image signal to be stored or transmitted can be kept low. With the image signal compressing and extending techniques, compression processing is carried out on a digital image signal in order to reduce its amount, and the compressed image signal is stored or transmitted. When an image is to be reproduced, a compressed image signal corresponding to the image is extended with extension processing. Thereafter, a visible image is reproduced from the extended image signal.
As on of methods for compressing image signals, an interpolation encoding method has heretofore been known as disclosed in, for example, U.S. Pat. No. 4,776,029. With the interpolation encoding method, image signal components of an image signal are classified into main components, which have been sampled at appropriate sampling intervals, and interpolated components other than the main components. The interpolated components are then subjected to interpolation prediction encoding processing based on the main components, i.e. the values of the interpolated components are predicted with the interpolation prediction from the main components. Thereafter, prediction errors between the predicted values and the actual values of the interpolated components are encoded into variable length codes, such as Huffman codes. In this manner, the image signal is compressed.
During the compression of an image signal, the image signal compressibility should be as high as possible. However, it is technically difficult to increase the compressibility markedly during the interpolation encoding. Therefore, in order for a high compressibility to be achieved, it is considered that component number decreasing processing, which results in a coarse spatial resolution, and the interpolation encoding be combined with each other.
If the number of the image signal components of an image signal is decreased and the spatial resolution becomes coarse, the image quality of a visible image reproduced from the resulting image signal will become bad. However, in the case of a certain type of image, for example, a radiation image of a human body, important image information falls within a low spatial frequency range. In such cases, even if the spatial resolution is made coarse and the high spatial frequency components are decreased, the image quality will not substantially become bad. Accordingly, in the case of such an image, the combination of the component number decreasing processing with the interpolation encoding can be employed during the compression of the image signal representing the image.
The component number decreasing processing and the interpolation encoding should be combined with each other such that the image quality of a reproduced visible image can be kept as good as possible and the signal compressibility can be kept as high as possible.